1. Field of the Invention
This invention relates to a process for fabrication of molecular sieving silica membranes suitable for use in gas separation applications.
2. Description of Prior Art
A gas separation membrane can be described as a semi-permeable organic or inorganic barrier capable of separating gases by virtue of differences in diffusion coefficients, solubility, or size. At high temperatures, facilitated transport mechanisms, such as selective adsorption or capillary condensation, are generally not operative. As a result, membrane selectivity is mainly achieved by size exclusion. Inorganic membranes, owing to their presumed superior thermal, mechanical, and chemical stability, compared to that of organic polymer membranes, have been proposed as replacements for energy intensive industrial separation processes such as distillation, drying, and evaporation. In addition to consuming less energy than conventional industrial separation processes, membrane systems are compact and modular, enabling easy incorporation into existing industrial processes. In order to be commercially feasible, gas separation membranes should exhibit high selectivity, which is achieved by having small pore sizes and narrow pore size distributions, and high permeance, which is defined as flux/pressure drop, achieved by having a large volume fraction porosity and a very thin selective layer. The selectivity, or separation factor, of the membranes pertaining to this invention is defined as the ratio of the permeance of the faster permeating gas to the permeance of the slower permeating gas. These membranes have potential applications in separations such as dehydrogenation, the separation of nitrogen from methane in ammonia plants, the production of oxygen and nitrogen from air, enhanced oil recovery, the separation of carbon dioxide and nitrogen from methane in natural gas processing and carbon dioxide recovery from land fills. To date, however, the potential applications of these membranes have not been realized due to the difficulty in obtaining very small pore sizes, and more importantly, due to the current procedures used for obtaining very small pore sizes which often result in an unfavorable reduction of the gas flux through the membrane.
Membranes for gas separation can be made using organic or inorganic routes. Organic membranes generally exhibit high separation factors for various gases of industrial interest, but their intrinsic permeability is very low. Gas transport through organic membranes occurs through a solution diffusion mechanism in which the permeation process is controlled by the molecular diffusion of gases in a dense organic polymer matrix. Several studies have shown that due to some intrinsic polymer property, such as free volume, there is an apparent trade-off between permeability and selectivity independent of chosen gas pair or polymer. See, for example, Robeson, L. M., Journal of Membrane Science, 62 (1991) pages 165-185. In addition to this trade-off, organic membranes have several other potential disadvantages including limited thermal stability, limited chemical stability, especially to organic solvents, and poor mechanical strength.
In contrast, porous inorganic membranes overcome many of the inherent limitations of organic membranes because there is no intrinsic relationship between permeability and selectivity. Permeability is controlled by volume fraction porosity, whereas selectivity is determined by the pore size and pore size distribution. Size-selective gas separation using porous inorganic membranes is by far the most attractive way to separate gas mixtures of industrial importance from the standpoint of energy consumption and economics. The combination of small pore sizes, narrow pore size distribution, high porosity, with tailored pore topology, pore surface chemistry, and surface adsorption/diffusion characteristics makes these membranes attractive for a wide range of applications including ultrafiltration, microfiltration, or gas separation, such as dehydrogenation, nitrogen/methane separation in ammonia plants, oxygen/nitrogen separation from air, enhanced oil recovery, separation of carbon dioxide and nitrogen from methane in natural gas processing, and carbon dioxide recovery from land fills. The increasing industrial requirements for low cost gases with high purity has provided a strong impetus towards developing inorganic membranes with unique separative properties. Inorganic membranes are prepared from both particulate and polymeric precursors with a wide range of pore sizes and porosities.
The particulate approach to preparing inorganic membranes involves slip-casting and calcination of a charged stabilized colloidal sol. See, for example, U.S. Pat. No. 4,562,021 which teaches a method of manufacturing a medium for microfiltration, for ultrafiltration, or for reverse osmosis in which a sol of particles of an oxide or a hydroxide of a chemical element is formed, a thickening agent is added to the sol, and the resulting sol is slipcast onto a support layer having pores which are larger than the pores desired for the filter medium, the thin layer deposited on the support medium being dried and then heat treated to eliminate the thickening agent and to sinter the particles of the deposited thin layer. See also, U.S. Pat. No. 5,096,745 which teaches a process for preparing particulate or polymeric titania ceramic membranes which includes the steps of preparing a colloidal solution containing a titanium organic salt with a specific ratio between water and titanium concentration in the colloid so as to determine whether the resulting membrane is either particulate or polymeric, adding to the colloidal solution an alkyl alcohol, and sintering the gel created from the colloid into a ceramic so as to prevent cracking of the resulting membrane. See also, U.S. Pat. No. 5,169,576.
Particulate sols consist generally of highly condensed ceramic particles in the 2-200 nanometer size range obtained in SiO.sub.2, Al.sub.2 O.sub.3, and TiO.sub.2 systems. In membranes prepared from monosized particulate sols obtained by hydrolysis of metal salts or alkoxides, pore volume depends simply on the particle packing, and pore size decreases linearly with the particle size when aggregation is avoided. An advantage of the particulate approach is that the porosity of the membrane is independent of pore size. However, the particulate approach has several disadvantages. In particular, colloidal stability is essential to avoid aggregation of the concentrating particles which otherwise would result in a bimodal pore size distribution, that is, pores within and between aggregates. In addition, the small particles necessary to obtain small pore sizes have associated with them a relatively thick tightly bound solvent layer that decreases the volume fraction of solids in the deposited film. The removal of this solvent during drying creates tensile stresses within the plane of the film that results in cracking.
The polymeric approach for preparing inorganic membranes involves slip-casting and calcination of a polymeric sol. Polymeric sols consist generally of more or less branched clusters that do not contain a fully condensed ceramic core and are obtained in the SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2 and TiO.sub.2 systems under conditions where the reaction rate is minimized; for the case of non-silicates, complexation chemistry is often used to reduce the polymer functionality. The inorganic polymer approach for making membranes offers several advantages. First, crack free membrane layers can be prepared where aggregation of polymeric precursors is exploited to control the pore size. Secondly, the size of the polymeric species can be controlled so that the deposited membrane forms a thin layer that spans the support with minimal pore plugging. And, finally, the physical and chemical characteristics of the membrane can be altered, either in the sol stage or in the deposited membrane stage, to alter the surface chemistry, while maintaining control of the pore size.
A potential disadvantage of the polymeric approach is that small pore sizes and narrow pore size distributions are achieved at the expense of pore volume. As a result, the permeability of the membrane may decrease to the point where the membranes are no longer practically viable.
U.S. Pat. No. 4,973,435 teaches a method for producing porous membranes of sinterable refractory metal oxides wherein a powder of the metal oxide is dispersed in an organic polymer in an amount such that, after the polymer has been carbonized in a subsequent step, there is a stoichiometrical excess of the oxide to carbon. The solution is then shaped to form a desired thin membrane and the polymer is then carbonized by heating it in a non-oxidizing atmosphere. The resulting product is heated to a temperature at which the carbon reacts with the oxide to form a volatile sub-oxide and carbon monoxide and the remaining oxide particles sinter together. A method of manufacturing porous sintered inorganic bodies with large open pore volumes in which a sinterable material in the form of finely ground powder is mixed with a leachable substance in the form of a powder, and a mixture of sinterable material and leachable substance is heated to a sintering temperature and maintained there until the sinterable mass is sintered, after which the mass is then cooled and the leachable substance leached from the sintered mass is taught by U.S. Pat. No. 4,588,540. U.S. Pat. No. 4,221,748 teaches a method for making porous, crushable cores having a porous integral outer barrier layer with a density gradient therein. The method includes the process steps of preparing a material composition consisting essentially of an organic binder, a reactant fugitive filler material, and an alumina flour. A portion of the material composition is then worked into a preform of a predetermined shape of the ceramic article to be produced. The preform is then heated to remove the organic binder while retaining substantially all of the reactant fugitive filler material therein. Heating is then continued in a controlled atmosphere to react the alumina and the reactant fugitive filler material to produce at least one or more suboxides of alumina. The one or more suboxides of alumina are vapor transported throughout the fired preform to produce a ceramic article having a predetermined porosity content, grain morphology, and crushability characteristics. A portion of the suboxides of alumina are oxidized to form a porous integral barrier layer of alumina at the surface of the ceramic article, the layer having a density gradient across its thickness. The remainder of the suboxides escape from the core resulting in a net weight loss. U.S. Pat. No. 3,963,504 teaches a porous ceramic monolithic structure prepared by shaping a ceramic filled polyolefinic material containing a plasticizer, shaping, extracting the plasticizer and treating to remove the polyolefin. Finally, U.S. Pat. No. 5,087,277 teaches a high temperature ceramic filter produced from a composition containing refractory cement, aggregate, pore forming additives and sintering agents.